More peanut acreage means more disease problems

It’s a simple equation — more peanuts being grown this year means more peanut disease problems.

“Whenever you increase peanut acreage, you have less room to rotate,” says Bob Kemerait, University of Georgia Extension plant pathologist. “And less room to rotate means more problems with soil-borne diseases, nematodes and leafspot.”

For a variety of reasons, peanut acreage has increased in the Southeast this year, with Georgia growers planning about a 20-percent boost, from 550,000 acres to more than 700,000, says Kemerait.

Many growers who are planting peanuts in a field for the first time want to know if they can “get away” with planting peanuts back to back, he says. But numerous research trials have shown a definite advantage to rotating between peanut crops, he adds.

“As you continue to have that crop in the field, the severity of soil-borne and leafspot diseases increases. And if that was all that happened, it wouldn’t matter so much. But coupled with an increase in disease is a loss in yield. The value of your peanut crop continually goes down as peanuts are planted in the same field year after year. That’s why rotation is so important. It doesn’t take very long for the value of your crop to drop after that first year of peanuts,” says Kemerait.

The severity of limb rot, stem rot or white mold, rhizoctonia limb rot and leafspot all increased when peanuts are grown continuously, he says.

In a three-year rotation study testing the severity of white mold in peanuts, bahiagrass proved to be the best rotational crop, says Kemerait. However, corn and cotton also are very good at reducing the level of white mold, he says.

“You can grow whatever you want in your ground, but you have an optimum situation in new ground, so why spoil it? Why not get into a good rotation? You might get away without rotating for a year or two, but do you really want to get away with it? Why not have a top peanut production program?”

Weather, says Kemerait, has a large impact on disease severity in peanuts. This past year, Georgia peanut producers saw a lot of tropical storms and hurricanes, he says.

“We know that in dry weather, you probably won’t have as much trouble with fungal diseases. In fact, you probably could spray less, especially if you use something like the AU Pnut Doppler radar system,” he says.

Several weather factors work against peanut producers, says Kemerait. The first is that wet weather favors the spread of disease, disease infection, and fungal growth.

“Also, wet weather very likely will delay you from spraying. It may rain so much that you can’t get into a field for a period of time. And if it rains too quickly after you apply a fungicide, you could have problems. You might get maximum soil-borne disease control, but you won’t get what you need on leafspot.”

If you spray a fungicide for soil-borne disease control, such as Abound, Folicur, Headline, Artisan, or Moncut, and the rain washes it off quickly, you might actually improve your white mold control, says Kemerait.

“That fungicide is washed from the foliage to the crown of the plant, and that’s a good thing for white mold. It’s not a good thing for leafspot. You’re also putting out those fungicides to control leafspot as well as soilborne diseases.”

Research has looked at the timing of peanut fungicides, or how long a fungicide needs to be on a plant before it is effective. We’re trying to collect data on this, but I have few rules of thumb.”

Headline, says Kemerait, probably requires the least amount of drying time. If it has been on the plant for an hour, or maybe even less, it probably will give good leafspot control, he says.

Chlorothalonil and Abound probably need to dry for two to four hours, says Kemerait, and research shows it may take even longer.

“For Folicur, we used to say four hours and then turn on your irrigation. Now, we’re thinking if you’re putting out just Folicur, you probably need 12 hours to get good leafspot and white mold control. If all you want from Folicur or any soil-borne program is white mold control, you can turn on your irrigation right after spraying and wash it down. However, most growers are relying on a single application for control of white mold and leaf spot.”

Many growers want to know if they need to spray again if they make their fungicide application, and then it rains soon after they leave the field, says Kemerait.

“You have several options. If you spray a fungicide and the rain comes fairly quickly afterwards, your first option is to ignore it and hope that it’s effective. Or, you could spray again to protect those leaves. The risk is that you might have spent time, effort and fuel on an extra fungicide treatment that you didn’t need.

“The third option is what I usually tell growers. If you had any drying time at all, except for the product that requires 12 hours, but you’re not sure you got the efficacy you need, rather than spraying in 14 days, you might shorten that interval and spray in about seven to 10 days.

There has never been a better time to grow peanuts, says Kemerait, because there now are more disease-resistant varieties available to producers. Also, there’s a broad variety of effective fungicides on the market.

In looking at leafspot diseases, early leafspot has been the dominant one since 2000, he says. In 2004, however, late leafspot was much more common in many areas of the Southeast than in previous years. In some areas, it was even more prominent than early leafspot.

“We distinguish between the two because late leafspot could be a bit more explosive than early leaf spot. If you have late leafspot in a field, you need to know that it’s probably producing more spores than the early leafspot with which you’re more familiar. If you add more spores in the right kind of weather, you’ll see a rapid increase in disease.”

There are two types of fungicides — protectant and systemic, says Kemerait. Protectants such as chlorothalonil products need to be on the leaf surface before infection occurs or before the spores infect the leaf, he says.

“Chlorothalonil products can be very effective, and we don’t have to worry too much about fungicide resistance, but they do have limitations. They must be in place, and you must have good timing. A grower can control leafspot effectively by using only these older fungicides, but the newer systemic fungicides offer additional benefits.

“They not only protect the leaf, but they’re able to move into the leaf as well. They also have some curative activity. For example, when you’re delayed getting in your field, or when you’re looking at an extended spray interval, you should spray a fungicide with these added benefits.”

All of these fungicides are very effective, says Kemerait. Since Folicur was labeled in 1994, it has been the backbone — until recently — of soil-borne programs. It also had excellent activity against leafspot, he says.

“In the past, it had been as good as Bravo on leafspot. But in the last couple of years, we have found that the level of control has dropped in some, but certainly not all, fields where it was used. In some research trials conducted in Georgia in 2004, the efficacy of Folicur on leafspot wasn’t what it used to be. We’re not sure why. It could be the weather, application timing, resistance, or some change in the chemistry. Bayer Cropscience, maker of Folicur, is aware of this issue and is taking the right steps to insure that Folicur remains a dependable part of our peanut fungicide arsenal.”

Growers who want to use Folicur have options, says Kemerait. “Some growers have heard that they don’t need to use Folicur unless they tank-mix it with another product for leafspot control.

“But if you have a good rotation, and conditions aren’t favorable for disease, then I don’t think you necessarily need to do that. If you’re at moderate to high risk to leafspot, which would include planting a more leafspot-susceptible variety, having a poor rotation, or having weather patterns that favor leafspot development, then I would consider tank-mixing Folicur with a leafspot control material. Or, you could introduce a product like Headline into a Folicur program to bolster leafspot control.”

There have been several changes in recent years in the control of soil-borne diseases of peanuts, says Kemerait. The first thing is improved varieties, which now have more resistance to soil-borne diseases.

“We have several good, effective fungicides. If you’re staying on a standard program, they’re all very effective. I believe management of soilborne diseases should be the foundation of your fungicide program. When you come up with a fungicide program for 2005, the first thing you should ask yourself is, what can I do to control white mold and limb rot? Soil-borne diseases — more than any other diseases — increase as rotation slacks off.”

Abound, he says, is good on white mold and is considered the best fungicide on limb rot. It also is labeled for the suppression of CBR. Moncut and Artisan are very similar, with good control against white mold and limb rot but not against CBR.

“Despite the leafspot concern, Folicur is an extremely good fungicide for white mold and limb rot control. It also is labeled for the suppression of CBR. Headline offers the best leafspot protection we’ve ever had, but that sometimes overshadows its soilborne effectiveness against white mold and limb rot. Headline also is now labeled for suppression of CBR.”

Metam sodium, he says, is the best material against CBR, applied as a fumigant at 10 gallons per acre prior to planting. Lorsban 15G, says Kemerait, is an insecticide that also has efficacy against white mold. Growers might want to consider using it in a fungicide program for possible increased control in fields where white mold is of particular concern, he says.